1. Field of the Invention
The present invention relates to a speed changing structure for a work vehicle.
2. Description of the Related Art
In conventional structures in agricultural tractors and other work vehicles, a travel speed changing device having a plurality of speed change positions is provided, and a manually operated speed changing lever and a travel speed changing device are mechanically linked by a linkage or the like. The travel speed changing device is not directly operated by the speed changing lever, but is configured so that control means operates the travel speed changing device on the basis of the operation of the speed changing lever by electrically detecting the operation position of the speed changing lever (e.g., see JP-A 2005-226726 and JP-A 2003-343712).
In JP-A 2005-226726, a hydraulic clutch-form speed changing apparatus (10 and 11 in FIG. 1) is provided in which a plurality of hydraulic clutches are combined, and also provided are control valves (31a, 32a, 33a, 34a, 38, and 39 in FIGS. 2 and 3) for controlling the travel speed changing device, a controller (see FIG. 2) for controlling the control valves, and a pushbutton-form shift-up button and shift-down button (61 and 62 in FIG. 2). The control valve is thereby operated by the controller each time the shift-up button and the shift-down button are pressed/operated, and the travel speed changing device is operated by the control valves toward the high speed side and the low speed side in single steps.
In JP-A 2003-343712, a synchromesh-form travel speed changing device (13, 14, 15, and 16 in FIG. 1) is provided together with a plurality of hydraulic cylinder-form actuators (35, 36, 37, and 38 in FIGS. 1 and 2) for operating the travel speed changing device, a controller (64 in FIG. 2) for controlling the actuators, and a speed changing lever (63 in FIG. 2). The actuators are thereby operated by the controller via operation of the speed changing lever, and the travel speed changing device is operated by the actuator in single steps.
In JP-A 2003-343712, in addition to a mode (see paragraph [0051]) that operates the travel speed changing device in single steps as described above, there is also a mode (see paragraph [0044]) in which the travel speed changing device is moved from a speed change position that corresponds to the operating position occupied prior to the movement of the speed changing lever to a speed change position that corresponds to the operating position occupied following the movement of the speed changing lever. The mode for operating the travel speed changing device in single steps and the mode for operating the travel speed changing device without passing though intermediate speed change positions are selected using a setting switch (65 in FIG. 2).
In JP-A 2003-343712, the change in running speed of the machine is relatively small when the travel speed changing device is operated by a single step from a current speed change position to the high speed side or the low speed side in the mode for operating the travel speed changing device in single steps. Therefore, the generated shock of the speed change is relatively low even if the travel speed changing device is repeatedly operated in single steps (consequently, some time is required for the travel speed changing device to reach the target speed change position).
In contrast, in JP-A 2003-343712, when the speed changing lever is moved to an operating position considerably separated from the current operating position (e.g., from first speed position to sixth speed position) in the mode for operating the travel speed changing device without passing through intermediate speed change positions, the travel speed changing device is operated without passing through the intermediate speed change positions, i.e., from the first speed position to the sixth speed position. Therefore, the change in speed of the machine is increased, and a relatively large speed change shock is generated (consequently, the travel speed changing device will reach the target speed change position in a short period of time).
In JP-A 2003-343712, a setting switch that is different from the speed changing lever is used to select the mode for operating the travel speed changing device in single steps and to select the mode for operating the travel speed changing device without passing through intermediate speed change positions. Therefore, after one of the modes has been selected using the setting switch, the operator may forget which mode was selected using the setting switch when the speed changing lever is operated after a short period of time has elapsed.
When, for example, the mode for operating the travel speed changing device without passing through intermediate speed change positions has been selected, but the driver mistakenly believes that the mode for operating the travel speed changing device in single steps has been selected, the driver may operate the speed changing lever to an operating position that is considerably far away from the current operating position, whereupon a relatively large speed change shock is generated as described above in conjunction with the operation of the travel speed changing device even though the mode for operating the travel speed changing device in single steps has been selected (and the driver may believe the generated speed change shock to be relatively small), and the driver may experience discomfort in such an unexpected situation.
An object of the present invention is to provide a speed changing structure for a work vehicle comprising a travel speed changing device that has a plurality of speed change positions, wherein the state in which the driver experiences discomfort in an unexpected situation is reduced in the mode for operating the travel speed changing device without passing through intermediate speed change positions in cases in which the apparatus can operate in a mode for operating the travel speed changing device in single steps and a mode for operating the travel speed changing device without passing through intermediate speed change positions.